Photostat or xerography is a means of copying documents, letters or pages of books without using liquid inks. This could be called ‘dry writing’.
A photostat machine makes use of static electricity. It relies upon the special properties of an element called selenium. When the light falls on selenium’s surface its electrical resistance drops sharply.
To copy a page, the operator places it face down, upon a horizontal glass window. The button is pressed on the photocopier and a bright light comes on to light up the page. Its image is projected onto a highly polished selenium-coated cylindrical drum through a lens. The drum is charged with static electricity.
The place where the light reflected from the white parts of the page falls on the selenium drum, the electrical resistance of the drum drops. Selenium’s charge leaks away to the ground. The light does not reach the drum from the black areas of the page. The drum on these areas retains the electrostatic charge. Now the drum is covered with a special powdered black ink. The ink adheres to the drum where there is still an electrostatic charge and so the image of the document gets formed in powdered ink on the selenium drum.
Now a sheet of plain white paper is pressed close to the drum. The paper develops a charge opposite to that of the drum by induction. This charge attracts the ink powder from the drum. The ink jumps from the selenium drum to the paper, thus transferring the image to the paper. The paper is heated before it leaves the machine. This melts the ink which sticks permanently to the paper, giving a reproduction of the original document.
In another type of electrostatic copiers, the image of the page is projected directly on to the paper, which charges its surface. The paper then passes through a bath of toner and the particles cling to the charged parts of the paper to produce the copy.